1. Field of the Invention
The present invention relates to an optical device which is suitable for use with an optical amplifier or the like in wavelength-division multiplexing (WDM) communication, and which has a variable optical attenuating function or a light-intercepting function.
2. Description of the Related Art
In WDM communication, multiwavelength signal light of a spectrum band of 1.55 μm emitted from a transmitting station is carried through one optical fiber, and is received by a receiving station, whereby high-capacity optical communication is carried out. In order to correctly receive the multiwavelength signal light at the receiving station, the power of the signal light of each wavelength that reaches the receiving station must substantially be the same. To achieve this requires a variable optical attenuator that compensates differences in loss in an optical fiber and in gain obtained by an optical amplifier in an optical transmission line with respect to the wavelengths of the signal light.
A conventional example of a variable optical attenuator is disclosed in Ref. 1 by Kawai et al. (The Inst. of Electronics, Inf. and Commun. Engineers (IEICE) Communication Society Convention, B-10-61, 1997). As shown in FIG. 1, the variable optical attenuator comprises a substrate 1, a Mach-Zehnder interferometer 12, and a heater 9 that is disposed on at least one of the arms of the Mach-Zehnder interferometer 12. The Mach-Zehnder interferometer 12 includes an optical circuit that comprises a core and a cladding that covers the core. The variable optical attenuator controls the amount of optical attenuation by adjusting phase differences between the two arms by thermal phase shifting.
The amount of optical attenuation by the variable optical attenuator is required to be equal to or greater than 35 dB, preferably equal to or greater than 40 dB. In the variable optical attenuator that makes use of a Mach-Zehnder interferometer, in order to increase the amount of optical attenuation to such values, two or more of the Mach-Zehnder interferometers 12, shown in FIG. 1, are connected in series (for example, in Japanese Patent Application Laid-Open No. S6-183406).
Further, in the variable optical attenuator in Ref. 1, the heating portion is only at one side of the core. Since the thermal expansion coefficient of the heater and that of the optical circuit are different, when thermal phase shifting is caused at the optical circuit, birefringence occurs at the same time. This increases polarization dependence of optical attenuation, so that optical performance is considerably reduced. For example, a typical polarization dependent loss (PDL) of a commercially available variable optical attenuator of the type disclosed in Ref. 1 is about 0.3 dB when the amount of optical attenuation is 10 dB, and is about 0.7 dB when the amount of optical attenuation is 15 dB. From the system side, a reduction in PDL to a value equal to or less than 0.2 dB over the entire operating range is expected.